Implants, such as dental implants, are well known in the art. They generally consist of a material, which is biocompatible and which additionally has a low elastic modulus and a high strength.
Apart from its biocompatibility and its mechanical properties, the osteointegrative properties of an implant are usually of major importance. The term osteointegration designates the direct structural and functional connection between living bone and the surface of the load-bearing implant. A good osteointegration means that the implant, after reaching a primary stability by screwing it into the bone, safely ossifies within a short healing time so that a permanent bond between implant and bone is obtained.
Dental implants which are currently in use are in general made of a metal, e.g. titanium, or a ceramic, e.g. a zirconia based ceramic, due to the biocompatibility and the favourable mechanical properties of these materials.
Alternatively to the materials mentioned above, it has further been suggested to use a material that closely resembles natural bone tissue, as an implant made of such a material would more fully integrate with existing bone tissue and enhance new bone growth around the implant. In this regard, the use of hydroxylapatite, which is very similar to the naturally occurring apatite, has been considered. Despite of the bioactive properties of implants formed from sintered hydroxylapatite, the mechanical properties of such implants have, however, turned out to be insufficient. For example, dental implants formed solely from hydroxylapatite are prone to crack and break after being implanted in a jaw bone.
Based on these observations, EP-A-0864332 suggests a method by which a coating of at least about 90% by weight of crystalline hydroxylapatite is applied on a metallic implant using plasma spraying.
Alternatively, U.S. Pat. No. 5,934,287 discloses a method by which particles formed of a material including hydroxylapatite are provided at a surface of a processed portion of a main body member formed of titanium or titanium alloy, such that each of said particles has a part embedded in said processed portion surface and a part protruding from said processed portion surface.
However, providing the surface of metallic implants with hydroxylapatite according to these methods has the disadvantage that the hydroxylapatite tends to be abraded when implanting the implant into the bone and subjecting it to physiological loading.
The development of the technology regarding osteointegrative properties of a metallic implant has thus gone off the coating of the surface with a bioactive material and has mainly focused on subtractive processes, as for example the one disclosed by EP-A-0388576.
Also with regard to the processing of the surface of a ceramic material, additive processes have been turned out to be problematic:
WO-A-2005/027771 relates to a process for preparing a dental installation in which a dispersion is applied on a substrate having a first porosity, said dispersion forming upon sintering a ceramic layer with a second porosity.
EP-A-0870478 relates to a dental retention element having a core of a high-strength material such as zirconia, said core being coated with a ceramic material which can be chemically and/or mechanically processed.
However, also the composite structures disclosed in WO-A-2005/027771 and EP-A-0870478 have the disadvantage that the ceramic coating is easily abraded.
A further alternative technique for providing a structure to the surface of a ceramic implant is described in DE-A-102006062712, which relates to a process in which the surface roughness is increased prior to the sintering by applying sharp-edged particles to the surface of the so-called green body and/or brown body. It is thereby preferred that the green body and/or the brown body is coated with a binder for fixation of the particles. According to another embodiment of the technique described in DE-A-102006062712, the particles are mixed with the binder and applied on the green body and/or the brown body.
DE-A-102006062712 thus teaches single particles to be applied on the surface of the implant body, which leads to protrusions in the shape of the particles. As mentioned above, it is according to DE-A-102006062712 essential that these particles are sharp-edged.
In order to prevent the implant to be damaged, DE-A-1020060621712 teaches that the particles are “trickled” on the green body and/or the brown body, respectively, without exerting pressure (“drucklos”). As illustratively shown in DE-A-1020060621712, a body with an interface between a basic body and the applied particles is thereby obtained.
The process according to DE-A-1020060621712 is however relatively complicated to perform. In particular with regard to the particles being trickled without exerting pressure, they tend to fall off the surface on which they are applied. Also, due to the fact that the trickled particles tend to accumulate in the valleys of the surface topography rather than on the peaks, a homogenous distribution of the particles is difficult to obtain.
Like for metallic implants, the development in providing an osteointegrative surface on a ceramic implant has thus gone off additive processes and has mainly focused on subtractive processes, as for example suggested by EP-B-1450722 and EP-A-1982670:
EP-B-1450722 relates to a dental implant made of zirconia ceramic which after abrasive blasting is subjected to a treatment using phosphoric acid, sulphuric acid, hydrochloric acid or mixtures thereof.
EP-A-1982670 relates to a process wherein a roughness is provided to the surface of the dental implant by sandblasting, milling and/or injection molding techniques prior to the etching of the implant with an etching solution comprising hydrofluoric acid.